In high-integrity applications such as avionics, traceability of each stage of implementation to a higher requirement is essential. A typical requirements statement might read:
<RR 2.1.2> The NP gauge shall graphically indicate the engine speed (input NPData) and indicates abnormal conditions by color change.To implement this requirement in a tool, graphics that represent the intended appearance of the HMI must be defined. This graphical content of an HMI may be represented as a tree composed of geometric elements. In the example in Figure 2, the pie shape is tied to an input called NPAngle, and the color of the pie is tied to a variable called NPColor. The textual readout is directly tied to the incoming NPData value.
The expected behavior of an HMI element can involve higher-level HMI dynamics that cannot be directly specified as transformations or attributed of the geometry itself. This might include mathematical expressions used to derive values for display, event- or state-based logic governing HMI behavior, or invocation of output behaviors based on user inputs. This may be referred to as presentation or backing logic and is often implemented through writing textual requirements followed by implementing code in a language such as C, C++, or Ada. Unfortunately, representing such logic as source code defeats the purpose of open architecture, because custom, untrusted code cannot be allowed to execute in an open-architecture certified avionics system. Another approach is required; one that supports logical specification and execution without code.
To specify the logic that governs the pie’s visual appearance and color change, logical transformations based on the input data must be defined. In order to do so, the HMI model must specify how the color of the element changes based on the input data. The structure of the logic tree allows one to specify a complex set of if/then/else conditions to fully specify each condition and the resulting actions.
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